A photovoltaic module, also known as a solar panel, is a device that converts the energy of sunlight directly into electricity by the photovoltaic effect. A photovoltaic module includes a plurality of photovoltaic cells, also known as solar cells, for example, crystalline silicon cells or thin-film cells. Stable photovoltaic energy production depends on many factors, some of which include specific semiconductor materials, interconnect technologies, and module encapsulation. To improve long term reliability, life testing can be performed on photovoltaic modules to determine their life cycle in the field, using various temperatures and biasing conditions. A module's open circuit voltage Voc (i.e., the maximum voltage from a cell when the net current through the module is zero), short-circuit current Isc (i.e., the maximum current from a cell when the voltage across the cell is zero), fill factor (i.e., the ratio of the maximum power from a cell to the product of Voc and Isc and cell efficiency can be observed under controlled environmental conditions in, for example, an accelerated, indoor light soaking chamber (or LSC).
During the testing process under controlled environmental conditions, photovoltaic modules to be tested are placed in a LSC preheated to a set point temperature and exposed to simulated light for extended periods of time. The set point is the temperature at which the control system of a LSC will hold the photovoltaic modules while under test. Conventional light soaking systems allow temperature control over a range from 50° Celsius to 90° Celsius. A solar simulator such as high power metal halide lamps, gas lasers, Xenon discharge lights or other light sources can simulate light exposure on the photovoltaic modules in the chamber. An illumination level of 100,000 lux of illumination over a 300 mm2 area is comparable to 1 sun light intensity exposure. Life testing can be performed with irradiance intensities in the range of 600 W/m2 to 1500 W/m2, however, other intensities could be employed. As an example of a test condition, life testing can be performed at so-called Standard Test Conditions (STC), which corresponds to a light intensity of 1000 W/m2 at 25° Celsius, with a reference solar spectral irradiance called Air Mass 1.5 (AM 1.5). The test duration typically ranges from 24 hours to 300 hours; however, other test &rations could be employed.
The overall photovoltaic module temperature and the temperature uniformity across the module need to be carefully controlled in order to accurately simulate module performance during life testing. Temperature uniformity of a photovoltaic module refers to the variation of the temperature over a surface of the module. Less variation in the temperature over the surface of the module results in a more accurate test. Moreover, the overall module temperature should be maintained as close to a predetermined set point temperature as possible.
Controlling the temperature and the temperature uniformity over the tested photovoltaic module is difficult with existing light soaking systems. Conventional light soaking systems, such as the one described in U.S. patent application Ser. No. 12/564,697 and filed on Jun. 30, 2009, primarily use fans to keep a constant air flow under and over the photovoltaic module during testing to maintain a set temperature uniformly over the module. Nevertheless, hot and cold spots on and within the module may still exist as it is difficult for such systems to effectively control the module temperature for set point temperatures below 50° Celsius. It is also difficult for such systems to achieve a temperature uniformity less than +/−7° Celsius across the photovoltaic module.
Accordingly, there is a need for a light soaking system that maintains better temperature uniformity across a photovoltaic module being tested and which can operate over a wider range of set point temperatures.